Pearl
<p>
Draw a single closed loop by connecting together the centres of
adjacent grid squares, so that some squares end up as corners, some as
straights (horizontal or vertical), and some may be empty. Every
square containing a black circle must be a corner not connected
directly to another corner; every square containing a white circle
must be a straight which is connected to <em>at least one</em> corner.
<p>
Drag between squares to draw or undraw pieces of the loop.
Alternatively, left-click the edge between two squares to turn it on
or off. Right-click an edge to mark it with a cross indicating that
you are sure the loop does not go through it.
